I have an OLD in-ground sprinkler system with a timer system that uses those old red diodes for the display, kinda like the ones you’d see on a calculator from the '70s. While I do drool over the Rachio and Rainbird setups, I decided to see what I could do with that old timer.
I set the dumb timer to run every zone sequentially every day at 7am. Then I wired in a relay (https://www.amazon.com/gp/product/B000LESCI2/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1) to break the common to the sprinkler solenoids. I use Webcore to log the rain every day, add it up, add in the forecasted rain over the next couple days, and only turn on the smart plug to the relay if the rain is below a certain threshold and the sprinklers haven’t run in the last ‘n’ days.
The relay essentially works like a typical rain sensor, except I have full control over whether it’s on or off. I can’t change the time of day or length of time the sprinklers run without walking all the way (gasp!) to the timer to change it, but that should be a “set it and forget it” kind of thing. I can choose to run the sprinklers more or less often by adjusting the number of days between runs or the trigger for the amount of rain.